Transcript weighted average atomic mass

9 Oct 2014
Agenda
Objectives:
Atomic number
Mass number
Isotopes
Average Atomic Mass
Slip Quiz 1
Isotopes – Are all atoms of an element alike?
Review Extension and Electrons in Atoms
Questions
Average Atomic Mass – POGIL/notes –
calculators/rulers
Homework
Slip Quiz Part 1
1. Define mass number of an atom and state
why it is called the “mass number.”
2. Which of the following pairs show 2 atoms
with the same number of neutrons? (Look
up and write in the appropriate atomic
numbers.)
37Cl and 38Ar
59Co and 61Co
32P
and 32S
65Zn
and 67Zn
Slip Quiz Part 1
1. Define mass number of an atom and state
why it is called the “mass number.”
Mass number is defined as the sum of the number
of protons and neutrons in the nucleus of the atom.
It is called mass number because the protons and
neutrons carry most of the mass of an atom, and so
the total number of them gives an idea of what the
mass of the atom itself will be.
Slip Quiz Part 1
Which of the following pairs show 2 atoms
with the same number of neutrons? (Look up
and write in the appropriate atomic
numbers.)
37Cl and 38Ar
59Co and 61Co
2.
17
32P
15
18
and 32S
16
27
65Zn
30
27
and 67Zn
30
Slip Quiz Part 1
Which of the following pairs show 2 atoms
with the same number of neutrons? (Look up
and write in the appropriate atomic
numbers.)
37Cl and 38Ar
59Co and 61Co
2.
17
32P
15
18
and 32S
16
27
65Zn
30
27
and 67Zn
30
Isotopes- Last Page
Electrons in Atoms and
Extension Questions
14. The number of electrons in each of the
atomic diagrams in Model 1 – is the same as
the number of protons in the nucleus of the
atomic diagrams (even if they are difficult to
spot.)
a. In a neutral atom, how does the number of
electrons compare to the number of
protons.
The number of electrons is equal to the
number of protons.
Isotopes
Electrons in Atoms and
Extension Questions
14b.
Atoms are “neutral” meaning they have no
overall electrostatic charge. The positive
charge in the nucleus caused by the
presence of protons is “cancelled” out
(when we view the atom as a whole) by
the same number of negatively charged
electrons in the electron cloud around that
nucleus. xp+ + xe- = 0
Isotopes
Electrons in Atoms andExtension Questions
16. Can two atoms with the same mass number ever
be isotopes of each other? Explain.
If two atoms have the same mass number,
they could be identical atoms with the same
atomic number and mass number. In this
case they would not be isotopes of each
other.
If two atoms have different atomic numbers
and just happen to have the same mass
numbers, then again they would not be
isotopes of each other.
Isotopes
Electrons in Atoms and Extension Questions
17. All models have limitations. What characteristics
of Model 1 are inconsistent with your understanding
of what atoms look like?
• Electrons are much smaller than protons
and neutrons yet in Model 1 they are all
represented by similar sized symbols.
• Electrons are much further from the
nucleus than represented in Model 1.
• Atoms and all the subatomic particles are
3Dimensional and not flat.
Average Atomic Mass
How are the masses on the periodic table determined?
Why?
Most elements have more than one naturally
occurring isotope. As you learned previously,
the atoms of those isotopes have the same
atomic number (number of protons), making
them belong to the same element, but they
have different mass numbers (total number
of protons and neutrons) giving them
different atomic masses.
Average Atomic Mass
How are the masses on the periodic table determined?
So which mass is put on the periodic table for
each element?
Is it the most common isotope’s mass? The
heaviest mass? This activity will help you
answer that question.
Qu 1 – 6 together.
7a. Expected number of atoms of
each isotope in a sample of 20
atoms
(whole numbers for atoms)
Mg-24: 78.99 x 20 =
100
Mg-24: 78.99 x 20 = 15.8 = 16
100
Mg-25: 10.00 x 20 = 2.0 = 2
100
Mg-26: 11.01 x 20 = 2.2 = 2
100
Is model 1 accurate in its representation of
magnesium at the atomic level? Explain.
8. If you could pick up an atom and
put it on a balance, the mass of that
atom would most likely be 23.9850
amu because there are more Mg-24
in the sample than any other isotope.
The chances are greatest that I would
pick up a Mg-24 atom.
9. a. From periodic table mass of Mg is
24.30amu (24.3050amu)
b. Does the decimal number shown on
the periodic table for magnesium
match any of the atomic masses listed
in Model 2?
No, none of the atomic masses listed
in Model 2 match the decimal mass
value reported on our periodic tables.
10. The periodic table does not show
the atomic mass of every isotope for
an element.
a) This would be an impractical goal
for the periodic table because it would
take up too much space to include
information about 2, 3 or more
isotopes for each element. (It would
be confusing to read also.)
10. The periodic table does not show
the atomic mass of every isotope for an
element.
b) It isn’t important to the average
scientist to have information about a
particular isotope of an element,
because in most cases a scientist will be
working with a mixture of isotopes and
their chemical behavior will be the
same. Information about individual
isotopes will not be necessary .
11. A practical way of showing the
mass of magnesium atoms on the
periodic table, given that most
elements occur as a mixture of
isotopes, would be to calculate an
average and report that.
12. A way to calculate the average
atomic mass of 100 magnesium
atoms.
12. A way to calculate the average
atomic mass of 100 magnesium
atoms.
Add up all the masses of all the
atoms in a sample and divide by the
number of atoms in the sample,
100.
Write in space on back
sheet.
Atomic Mass Units
(amu)
The masses of protons, neutrons and
electrons in g are very small and difficult to
work with.
Chemists developed a method of measuring
the mass of an atom relative to the mass of a
specifically chosen atomic standard.
Carbon-12 atom
One carbon-12 atom was assigned a mass of
exactly 12 atomic mass units (amu).
Atomic Mass Units
(amu)
…atomic standard
Carbon-12 atom
One carbon-12 atom was assigned a mass of
exactly 12 atomic mass units (amu).
One atomic mass unit (amu) is defined as
1/12th (one twelfth) of the mass of a carbon12 atom.
1 amu is close to the mass of a single proton
or single neutron. See p 102 Table 4-2
Model 3 – Proposed Average Atomic Mass Calculations
Mary’s method gives same answer as PT
(78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu)
100
= 24.305 amu
Jack’s Method – gives same answer as PT
(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu)
= 24.305 amu
Alan’s method – assumes equal representation of all
isotopes, only finds average for 3 atoms, one of each
isotope
(23.9850 amu) + (24.9858 amu) + (25.9826 amu) = 24.984 amu
3
14.c. show Mary and Jack’s methods are
mathematically equivalent models.
Mary’s method
(78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu)
100
=78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu)
100
100
100
=(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu
=(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu)+ (0.1101)(25.9826amu)
= 24.305 amu
Jack’s Method
I will look at your answer to questions 15. when you
turn in the packet at the end of class or at start of next
class meeting.
Read This!
Recall that all isotopes of an element have
the same physical and chemical properties,
with the exception of atomic mass (and for
unstable isotopes, radioactivity). Therefore,
the whole periodic table lists a weighted
average atomic mass for each element. In
order to calculate this quantity, the natural
abundance and atomic mass of each isotope
must be provided.
16. Consider the individual atomic
masses for magnesium isotopes give in
Model 2.
a) Which isotope has an atomic mass
closest to the average atomic mass listed
on the periodic table?
16. Consider the individual atomic
masses for magnesium isotopes give in
Model 2.
a) Which isotope has an atomic mass
closest to the average atomic mass listed
on the periodic table?
24Mg
b) Give a mathematical reason for answer to
part a.
Weighted average atomic mass cont.
24Mg
is the most common isotope and is thus
most heavily “weighted” in the equation for
average atomic mass.
There are more 24Mg atoms in a sample than
atoms of the other isotopes since it is the
most naturally abundant isotope. The mass
of 24Mg atoms contribute the most to the
average mass of the sample.
Weighted average atomic mass cont.
17. Boron has two naturally occurring isotopes:
boron-10 and boron-11. Which isotope is more
abundant on Earth? Use grammatically correct
sentences to explain the answer.
(3 mins to discuss) – share.
Weighted average atomic mass cont.
The expected mass of boron-10 would be 10
amu and the expected mass of boron -11
would be 11 amu.
On the periodic table the average atomic mass
for boron is listed as 10.811amu, which is
numerically closer to 11 than it is to 10.
Therefore we can conclude that boron-11 is
more abundant on Earth than boron-10.
Homework
Turn in assignment using our Google Classroom by 11:59pm Tues Oct.
14th.
• Find out the names that 110, 111, 112, 114, and 116
have now been given. What is the latest news about
element 118?
• Who/what makes the decisions about element names?
• How long does it take for a name to be decided upon?
• Record your source(s) using MLA formatting.
Read your notes about Chromatography for lab Mondayshoes – markers to investigate.